The present invention relates to a television video signal synchronizing apparatus which synchronizes an input television video signal with a reference timing signal.
A television video signal synchronizing apparatus (frame synchronizer) is ordinarily used to synchronize a television video signal transmitted from one television broadcasting station with a reference synchronizing timing of another television broadcasting station where the television video signal synchronizing apparatus is located, and the technique for synchronizing a television video signal is disclosed in, for example, the U.S. Pat. Nos. 3,909,839 and 4,007,486.
Recently, within a single television broadcasting station, such frame synchronizer has been frequently employed in order to cancel a slight phase difference (synchronous difference) between video signals delivered from different video sources such as signals from a studio room and a VTR room. The compensation to cancel the slight phase difference is required when a plurality of video signals which are delivered from the respective video sources, are composed (combined) to make a special effect television picture, for example. The use of a frame synchronizer in this way is referred to as a common synchronism use, and will be so referred to hereinafter.
When the frame synchronizer is used for the common synchronism use, i.e., used for cancelling the slight synchronous difference from sources in the same television broadcasting station, the output video signal delivered therefrom is unavoidably delayed from its input video signal by about one frame period (33 m sec in the NTSC system). Therefore,the output video signal from the frame synchronizer has a delay of about one frame period in comparison with an audio signal. The time delay between the output video signal and the audio signal is called "lip-lag" and, therefore, causes a serious phenomenon in which the motion of an announcer's lip on a picture monitor and his voice are not synchronous with each other. The one frame time delay is caused as follows. In the frame synchronizer, the input video signal is processed through low-pass filtering, analog-to-digital (A/D) converting and serial-to-parallel data converting prior to being stored in a frame memory during a writing phase. In consequence, delays of 595 n sec, 70 n sec and 1680 n sec in such respective processes, i.e., 2,345 .mu.sec of total delay, for example, occur. On the other hand, delays of 70 n sec due to digital-to-analog (D/A) converting and 595 n sec due to low-pass filtering, i.e., 0.665 .mu.sec of total delay occur during a reading phase. A delay of 840 n sec is further derived when an inverter for a chrominance component is arranged between the frame memory and a D/A converter. In this case, a total delay of 1.505 .mu.sec occurs at the reading side. Therefore, a total delay of at least 3.85 .mu.sec, which represents an offset delay unavoidably occurs when a television video signal is transmitted through the frame synchronizer. In the common synchronism mode of use of the frame synchronizer, the slight phase difference between the input video signal and the reference timing (the output video signal) is, of course, less than 3.85 .mu.s and, therefore, the reading operation is controlled to be delayed by about one frame period after the writing operation in order to accurately synchronize the input video signal with the reference timing. In consequence, the undesirable "lip-lap" is introduced between the output video signal and the audio signal.
Conventionally, an audio signal delay apparatus is provided in order to compensate for the above-mentioned "lip-lag". However, the audio signal delay apparatus for one frame period is considerably large in size and involves complicated hardware.